One example of an energy conversion device having fluorescent concentrators is found in U.S. Pat. No. 6,538,191 to Stuart, entitled PHOTOCELL WITH FLUORESCENT CONVERSION LAYER, which is incorporated herein by reference. Typically, an energy conversion device having a fluorescent concentrator includes a top layer of material that is transparent to light, a bottom layer, and fluorophores that are distributed in the volume of the structure. These components, forming the fluorescent concentrator, are arranged such that light rays that are incident on the top surface of the structure reach the fluorophores. The fluorophores absorb the light and re-emit it at a different wavelength. The light rays that are emitted by the fluorophores then propagate to the edges of the fluorescent concentrator. The structure is typically large to collect a lot of light, and strips of solar cells are placed at the edges of the fluorescent concentrator and convert the fluorescence into electrical energy.
The concentrator depends on total internal reflection of the fluorescence for it to reach the edges of the fluorescent concentrator, and since the fluorescence may be absorbed and re-emitted many times by the fluorophores in the concentrator and fluorescent emission occurs at all angles with respect to the surface of the structure, this can contribute to large losses in the concentrator.
Another way of constructing a concentrator for electromagnetic energy is to use photonic band-gap materials instead of traditional dielectrics for the structure. In these materials, radiation in a certain range of frequencies does not propagate, so they transmit a wide range of frequencies (pass-band) while prohibiting other frequencies. FIG. 1 shows a band-gap diagram for a typical photonic band-gap structure. Plotted is frequency ω 102 versus wave vector k 104. For this structure there is a photonic band-gap 106 and in this range of frequencies radiation does not propagate. Light in frequency ranges above and below the band-gap 106 propagates within the photonic crystal. Photonic crystals may have more than one band-gap and pass-band.